smooth-muscle cells compared to skeletal muscle

Which of the following is absent in smooth-muscle cells compared to skeletal muscle cells?

• a.Troponin
• b.Calmodulin
• c.Calcium
• d.Myosin light-chain kinase
• e.Actin and tropomyosin interactions

 The answer is (a).
Smooth muscle is the least specialized type of muscle and contains no troponin. The contractile process is similar to the actin-myosin interactions that occur in motility of nonmuscle cells. In the smooth-muscle cell, actin and myosin are attached to intermediate filaments at dense bodies in the sarcolemma and cytoplasm. Dense bodies contain alpha-actinin and, therefore, resemble the Z-lines of skeletal muscle. Contraction causes cell shortening and a change in shape from elongate to globular. Contraction occurs by a sliding filament action analogous to the mechanism used by thick and thin filaments in striated muscle.

The connections to the plasma membrane allow all the smooth-muscle cells in the same region to act as a functional unit. The sarcoplasmic reticulum is not as well developed as that in the striated muscles. There are no T tubules present; however, endocytic vesicles called caveolae are believed to function in a fashion similar to the T tubule system of skeletal muscle. When intracellular calcium levels increase, the calcium is bound to the Ca2+ -binding protein, calmodulin. Ca2+-calmodulin (answers b and c) is required and is bound to myosin light-chain kinase (answer d) to form a Ca2+ -calmodulin-kinase complex. This complex catlayzes the phosphorylation of one of the two myosin light chains on the myosin heads. That phosphorylation allows the binding of actin to myosin. A specific phosphatase
dephosphorylates the myosin light chain, which returns the actin and myosin to the inactive, resting state. The actin-tropomyosin interactions (answer e) are similar in smooth and skeletal muscle.
Smooth-muscle cells (e.g., vascular smooth-muscle cells) also differ from skeletal muscle cells in that like fibroblasts, they are capable of collagen, elastin, and proteoglycan synthesis.